Paxton was diagnosed with breast cancer two years ago, and underwent chemotherapy almost immediately, KCAL reported.

"It was like the world stopped," her older sister, Morgan Williams, 33, told InsideEdition.com. "I didn’t have time to worry about me — I just wanted to worry about them. It was probably the worst thing she’s ever experienced in her life."

When the family discovered she would not be able to have her own children as a result of the treatment, Williams, who already has a 9-year-old daughter, didn’t hesitate to step in.

"I didn’t really think about it, I just knew my sister always wanted to be a mom, so when she was told she couldn’t carry her own children, [I replied], 'I’ll carry. I’ll be your surrogate,'" Williams said.

Even though Williams never thought about the promise she made to her sister in that moment, Paxton said she wasn’t at all surprised by her older sister’s generosity.

"She has that step-in and go personality," Paxton told InsideEdition.com. "She couldn’t protect me from the cancer. By doing all that, it’s how she felt she could protect."

Paxton explained that her sister was present every step of the way as she received her treatment. From setting up a fundraiser to help pay off medical bills to bringing her and her husband dinner every night.

"It was non-stop how much she helped us," she said.

After Paxton completed treatment months after, they implanted the embryos, and were delighted to find that Williams was carrying twins.

Williams gave birth to the girls last Monday, and she said her daughter is already looking forward to be an older cousin to the twins.

“My daughter already wants to be over there every day, helping and changing diapers," she said. "Out of something that’s so terrifying and so scary, there’s something so beautiful and miraculous that’s come out of it. Now they have two beautiful girls, and the fun begins."